(a) Field of the Invention
The present invention relates to a method of timing acquisition and carrier frequency offset estimation for an orthogonal frequency division multiplexing (OFDM)-based communication system. Particularly, the present invention relates to an apparatus for acquiring OFDM symbol timing and estimating a carrier frequency offset by applying a conjugate complex symmetric OFDM symbol in a time domain to a timing acquisition and frequency offset estimation algorithm in an OFDM-based communication system, and a method thereof.
(b) Description of the Related Art
A fourth mobile communication system that requires data transmission of large capacity, such as a wireless local area network (WLAN), wireless broadcasting, or digital multimedia broadcasting (DMB) employs an orthogonal frequency division multiplexing (OFDM) technique for transmission of wideband high-speed data. The OFDM technique is a multi-carrier transmission technique that divides a range of available bandwidth spectrum into a plurality of subcarriers. According to the OFDM technique, a series of input data rows are converted to N parallel data rows, each being transmitted in the divided subcarrier.
FIG. 1 shows a structure of a frame used for data transmission in an OFDM-based mobile communication system.
Data transmitted in the OFDM-based mobile communication system is formed of a preamble and data symbols that contain subsequent data.
The preamble includes information on frame synchronization, cell search, time/frequency synchronization, and channel estimation. The preamble is typically located at a start point of every frame, but it may be located in a middle portion or at an end portion of each frame.
FIGs. 2-1 and 2B show exemplary preamble sequence allocation when each cell transmits data by using a different subcarrier in an OFDM system.
FIG. 2A exemplarily shows segments used by the respective cells in IEEE 802.16, and there are three types of segments: segment a, segment b, and segment c.
FIG. 2-2 shows a subcarrier in which a preamble is used in the cell structure of FIG. 2-1.
As shown in FIG. 2-2, a signal transmitted from a base station is formed by binary phase shift keying (BPSK) modulation of a preamble pattern that is allocated to the corresponding cell or segment of a subcarrier arranged in an interval of three subcarriers in a frequency domain.
Each cell can have a maximum of three segments, and arrangement of available subcarriers in each of the three subcarriers may have a different offset in accordance with a segment ID.
That is, a subcarrier index in cells (e.g., Cell 0, Cell 2, Cell 4, Cell 6) with segment ID of a is a multiple of 3 starting from 0 (i.e., 0, 3, 6, 9, . . . ), a subcarrier index in cells (e.g., Cell 1) with segment ID of b is a multiple of 3+1 starting from 1 (i.e., 1, 4, 7, 10, . . . ), and a subcarrier index in cells (e.g., Cell 3 and Cell 5) with segment ID of c is a multiple of 3+2 starting from 2 (i.e., 2, 5, 8, 11, . . . ), and the subcarriers are used for preamble pattern transmission.
Therefore, a preamble signal of each segment has the same pattern repeated three times during a single OFDM symbol period in a time domain. In this case, the respective repeating patterns have a phase rotation of ±2π/3 therebetween in accordance with an arrangement of available subcarriers. By using characteristics of this repetition, an absolute value of auto-correlation having a time interval of a ⅓ OFDM symbol period for a received signal can be observed such that the presence of a preamble can be easily detected, and at the same time, timing acquisition for the preamble can be achieved. In addition, a carrier frequency offset can be estimated within a ±½ range of a frequency difference between subcarriers by observing a mean phase difference between the repeating patterns.
When estimating a carrier frequency offset, an algorithm that uses a cyclic prefix (CP) for data symbols transmitted after the preamble can be applied. However, a signal received at a terminal located at an area where three different segment signals, each having different segment IDs, can be received, does not have the repeating pattern of the time domain since all available subcarriers of the signal are modulated. Therefore, a conventional algorithm cannot achieve appropriate performance in frame timing acquisition and carrier frequency offset estimation.
In order to solve the above-stated problem, Korean Patent No. 10-2001-0050104 (entitled “Method for creating Symmetric-Identical preamble and method for synchronizing symbol and frequency of Orthogonal Frequency Division Multiplexed signals by using the Symmetric-Identical preamble”) has disclosed a method for symbol timing and frequency synchronization for an OFDM signal by using a symmetric-identical preamble. In addition, Korean Patent No.10-2003-0097867 (entitled “Method for embodying frame preamble in wireless communication based on OFDM, and method for acquiring frame synchronization and searching cells using the preamble”) has disclosed a method for frame synchronization and cell searching by providing an initial synchronization algorithm with a new preamble structure, and Korean Patent No. 10-2000-0032951 (entitled “Optimized synchronization preamble structure”) has disclosed a specific preamble pattern with optimized PAPR and correlation characteristics. However, since the respective three prior arts use a preamble in a specific structure or an independently designed preamble, the three prior arts still have a problem in frame timing acquisition and carrier frequency offset estimation by using a typical OFDM symbol.
In addition, a transaction (entitled “Preamble structure for single frequency cellular systems using orthogonal frequency division multiplexing”) published in the IEEE Transactions on Consumer Electronics in Feb. 2004 proposes a novel preamble structure including frame synchronization acquisition, carrier frequency acquisition, and cell search. However, this transaction also provides a method using a novel preamble structure, and therefore it is difficult to achieve frame timing acquisition and carrier frequency offset by using this method.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.